1. Field of the Invention
This invention relates to a method for the purification of waste water. Particularly, this invention relates to a method for the purification of waste water containing chemical oxygen-demanding substances (hereinafter referred to as "COD component") by wet oxidation. More particularly, this invention relates to a method for effective purification of waste water containing a COD component, i.e. harmful oxidizable organic or inorganic substances (herein after referred to "impurity"), which method effects the purification of the waste water by subjecting the waste water to wet oxidation in the presence of molecular oxygen thereby converting the organic substances into harmless compounds such as carbon dioxide, water, and nitrogen.
2. Description of the Prior Art
Among the methods currently available for the treatment of waste water, the biochemical method called the activated sludge method and the wet oxidation method called the Zimmermann method have been renowned.
For the wet oxidation method, use of a varying oxidizing catalyst for the purpose of heightening the reaction rate has been proposed. Further, for the wet oxidation method, no matter whether a catalyst is absent from or present in the reaction site, the single cylinder type reaction column is used as the reaction vessel.
The activated sludge method consumes a long time for the decomposition of organic substances and requires the waste water to be diluted to a concentration fit for the growth of algae and bacteria and, therefore, has the disadvantage that the facilities for the treatment of activated sludge occupy a large floor space. Further, in recent years, the handling of the surplus grown sludge has been entailing an immense expenditure particularly in urban districts. The Zimmermann method comprises oxidatively decomposing organic substances in an aqueous solution by introducing air into the aqueous solution of the organic substances under a pressure in the range of 20 to 200 atmospheres at a temperature in the range of 200.degree. to 370.degree. C. Since the reaction rate is low and the decomposition consumes a long time, this method necessitates a large reaction vessel made of a highly durable material and attains no real economy because of expensive equipment and expensive operation. Since the liquid phase within the reaction vessel cannot be retained when the reaction temperature is elevated by the heat of reaction, this method has the disadvantage of being incapable of effectively treating waste water whose COD component has a high calorific value. Also for this method, use of a varying oxidizing catalyst for the purpose of heightening the reaction rate has been proposed. For the use of such an oxidizing catalyst, none of the conventional methods for waste water treatment is specifically devised to relieve the reaction vessel of the heat of reaction.
Particularly in the treatment of highly concentrated by the conventional wet oxidation method, the amount of the heat generated by the reaction is conspicuously large. Since the liquid phase within the reaction vessel cannot be retained when the temperature thereof is elevated in consequence of the increase in the heat of reaction, it becomes necessary to curb the amount of generated heat by diluting the waste water under treatment. The dilution has as a problem the liability to increase the amount of waste water to be treated. Even where the waste water has a low COD content and the amount of the heat generated by the reaction is small, the heat generated still goes to elevating the liquid temperature and inevitably requires the reaction pressure to be unduly increased for the purpose of keeping the waste water in a liquid state. Thus, the treatment involving the use of an oxidizing catalyst attains no real economy because of an addition to the expense of equipment and that of operation.
An object of this invention, therefore, is to provide an improved method for the purification of waste water.
Another object of this invention is to provide method capable of effective purification of waste water containing a COD component, i.e. harmful organic or inorganic substances, by subjecting the waste water to wet oxidation in the presence of molecular oxygen thereby converting the harmful substances into such harmless compounds as carbon dioxide, water, and nitrogen.